In ion mobility spectrometry molecules for analysis are ionised and subsequently carried to a shutter due to a general potential difference in the spectrometer. By applying a reverse potential difference between the first electrode elements and the second electrode elements, that is to say a potential difference that is opposite to the general potential difference, the shutter is closed and prevents the ions from passing through towards the collector plate of the spectrometer. If, on the other hand, a forward potential difference is applied over the first and second electrode elements, that is to say in the same sense as the general potential difference in the spectrometer, the shutter is opened and allows the ions to pass through and continue further on their way in the direction of a collector plate.
When the potential difference over the first and second electrode elements is briefly switched from reverse to forward then back to reverse, that is to say the shutter is briefly opened then closed, short bursts of ions are emitted in the direction of the collector plate through the so-called drift space.
An electric field or drift potential is applied over this drift space between the shutter and the collector plate, whereby the ions will migrate in the direction of the collector plate. Since different types of ion have a different displacement velocity within the drift potential, this being referred to as ion mobility, a swarm of one type of ions will arrive at the collector plate at a different time from a swarm of another type.
On the basis of the time taken by a swarm of ions to move from the shutter to the collector plate, also referred to as the drift time, it is possible to determine which type of ions, and therefore which molecules, are involved.
The drawback of the known shutter is however that, when the shutter is briefly opened and closed again, a relatively elongate swarm of ions is ejected in the direction of the collector plate. In order to enable measurement of the difference in drift times between the different types of ions it is necessary for the swarms of different ions to be wholly pulled apart over the length of the drift space as a result of the specific ion mobility. Because the swarm of ions ejected through the shutter is elongate, a considerable length is thus required for the drift space. This length usually amounts to at least about 4 to 20 centimeters.
An additional drawback of this length of the drift space is that the housing of this space must comply with highly specific design requirements in order to obtain a uniform potential difference through the space.
Another drawback of the known shutter is that the shape of the ejected swarm of ions has an irregular form. This shape resembles to some extent the shape of a stingray. The detection curve of a swarm of specific ions on the collector plate will hereby be erratically shaped, whereby it is more difficult to distinguish different drift times of the different swarms of ions from each other.
All these above stated drawbacks make it difficult to reduce the size of known ion mobility spectrometers.